Note: Descriptions are shown in the official language in which they were submitted.
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CREAVIS Gesellschaft fur Technologie O.Z. 5604
unG Innovation mbH
Polyelectrolyte coated permeable composite material,
its preparation and use
The present invention relates to a polyelectrolyte
coated permeable composite material and to its
preparation and use.
Permeable composite materials have diverse possible
applications. Materials of this kind are especially
suitable for use as membranes.
Membranes for separating, say, ethanol/water mixtures
by pervaporation have been adequately described in the
literature. Products available commercially are based
on membranes having a multilayer construction. They
consist of a highly porous polymer support structure
(usually a polyacrylonitrile membrane on a polyester
nonwoven) to which a crosslinked polyvinyl alcohol
layer has been applied. This layer usually possesses a
thickness of a few micrometers.
Further polymers suitable for preparing a selective top
layer are block copolymers of polyols and poly-
urethanes. Recent times have also seen increasing use
of inorganic materials. Among these, mention may be
made in particular of membranes having zeolite top
layers and also silica layers. Composite materials such
as zeolite filled polysiloxanes have also been
investigated in detail (R.Y.M. Huang (Ed.),
"Pervaporation Membrane Separation Processes",
Elsevier, Amsterdam 1991).
Moreover, the use of polyelectrolyte layers as
selective layers in membranes has been frequently
described in the literature (K. Richau, H.-H. Schwarz,
R. Apostol, D. Paul; J. Membr. Sci. 113, (1996) 31,
Sang Yong Nam, Young Moo Lee; J. Membr. Sci. 135 (1997)
161 and P. Stroeve; V. Vasquez; M.A.N. Coelho;
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J.F. Rabolt; Thin Solid Films 284/285 (1996) 706).
Particularly the method of preparing self-organized
polyelectrolyte layers, as has been proposed by a
number of authors (F. van Ackern; L. Krasemann;
B. Tieke; Thin Solid Films 327-329 (1998) 762 and
L. Krasemann; B. Tieke; J. Membr. Sci. 150 (1998) 23),
is extremely suitable for preparing particularly thin
layers. Since the flow through a membrane is in inverse
proportion to the layer thickness of the membrane, a
high flow can be achieved through such a membrane.
Such polyelectrolyte layers are normally deposited on
polyacrylonitrile supports activated by plasma
treatment, as also used for polyvinyl alcohol
membranes.
EP 0 472 990 describes the deposition of poly-
electrolytes as a monolayer on symmetrical organic or
inorganic surfaces which are not permeable and
therefore cannot be used as membranes.
All of these membrane systems have a number of
disadvantages. The polymer membranes and the zeolite
filled polymer membranes lack the temperature stability
required to achieve consistent separations at
temperatures above 80°C. The zeolitic and silica coated
inorganic membranes, which operate very well at higher
temperatures, are correspondingly expensive and of
scant commercial availability. Moreover, they are
highly susceptible to acidic media, which destroy the
selective layers of these membranes within a few
minutes or a few hours. Additionally, the inorganic
membranes are generally inflexible and are therefore
easily destroyed under tensile or torsional stress.
It is an object of the present invention, therefore, to
provide a pervaporation membrane which provides good
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separations and is durable at relatively high
temperatures and/or at a pH < 7.
It has surprisingly been found that polyelectrolyte
layers may be deposited not only on organic support
material or on symmetrical surfaces but also on
inorganic - including ceramic - permeable surfaces. A
polyelectrolyte coated permeable composite material of
this kind, based on at least one perforate and
permeable support comprising on at least one side of
the support and in the interior of the support at least
one inorganic component comprising substantially at
least one compound of a metal, semimetal or mixed metal
with at least one element from main groups 3 to 7, may
be used as a pervaporation membrane even at relatively
high temperatures and at a pH < 7.
The present invention accordingly provides a permeable
composite material based on at least one perforate and
permeable support comprising on at least one side of
the support and in the interior of the support at least
one inorganic component comprising substantially at
least one compound of a metal, semimetal or mixed metal
with at least one element from main groups 3 to 7,
wherein the composite material carries at least one
polyelectrolyte layer on the inner and/or outer
surfaces.
The present invention likewise provides a process for
preparing a composite material as claimed in at least
one of claims 1 to 14, which comprises coating a
composite material which has surface charges and is
based on at least one perforate and permeable support
comprising on at least one side of the support and/or
in the interior of the support at least one inorganic
component comprising substantially at least one
compound of a metal, semimetal or mixed metal with at
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least one element from main groups 3 to 7, at least
once with a polyelectrolyte.
The present invention additionally provides for the use
of a composite material as claimed in at least one of
claims 1 to 14 as a membrane for separating
alcohol/water mixtures, especially ethanol/water
mixtures.
The polyelectrolyte coated composite material of the
invention is highly suitable as a membrane for
pervaporation. Owing to the particular structure of the
polyelectrolyte coated composite material of the
invention, membranes of particular chemical and thermal
stability are obtained which also exhibit very high
flow rates and separation factors.
The composite material of the invention is described by
way of example below, without being restricted thereto.
The permeable composite material of the invention based
on at least one perforate and permeable support
comprising on at least one side of the support and in
the interior of the support at least one inorganic
component comprising substantially at least one
compound of a metal, semimetal or mixed metal with at
least one element from main groups 3 to 7 carries at
least one polyelectrolyte layer on the inner and/or
outer surfaces. By the interior of a support is meant,
for the purposes of the present invention, cavities or
pores in a support.
In accordance with the invention, the perforate and
permeable support can have interstices with a size of
from 5 nm to 500 ~,m, preferably with a size of from
50 nm to 50 Vim, and with very particular preference
with a size of from 50 nm to 5 ~tm. The interstices may
be pores, meshes, holes, crystal lattice interstices,
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or cavities. The support may comprise at least one
material selected from carbon, metals, alloys, glass,
ceramics, minerals, plastics, amorphous substances,
natural products, composites, or of at least one
combination of these materials. The supports which may
comprise the aforementioned materials may have been
modified by a chemical, thermal or mechanical treatment
method or by a combination of treatment methods.
Preferably, the composite material comprises a support
comprising at least one metal, natural fiber or polymer
which has been modified by at least one mechanical
deformation technique or treatment method, such as
drawing, compressing, flexing, rolling, stretching or
forging, for example. With very particular preference,
the composite material comprises at least one support
comprising at least woven, bonded, felted or
ceramically bound fibers, or at least sintered or
bonded moldings, beads or particles. In a further
preferred embodiment, a perforated support may be used.
Permeable supports may also be those which acquire
their permeability, or have been made permeable, by
laser treatment or ion beam treatment.
It may be advantageous for the support to comprise
fibers of at least one material selected from carbon,
metals, alloys, ceramics, glass, minerals, plastics,
amorphous substances, composites and natural products
or fibers of at least one combination of these
materials, such as asbestos, glass fibers, carbon
fibers, metal wires, including steel wires, rock wool
fibers, polyamide fibers, coconut fibers, and coated
fibers, for example. It is preferred to use supports
which comprise woven fibers of metal or alloys. Wires
may also be used as metal fibers. With very particular
preference, the composite material comprises a support
comprising at least one woven fabric made of steel or
of stainless steel, such as woven fabrics produced from
steel wires, steel fibers, stainless steel wires or
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stainless steel fibers by weaving and having a mesh
size of preferably from 5 to 500 ~,m, with particular
preference from 5 to 50 or from 50 to 500 ~,m, and with
very particular preference from 70 to 120 Vim.
Alternatively, the support of the composite material
may comprise at least one expanded metal having a pore
size of from 5 to 500 N.m. In accordance with the
invention, however, the support may also comprise at
least one particulate sintered metal, a sintered glass
or a metal nonwoven having a pore size of from 0.1 ~.m
to 500 Nxn, preferably from 3 to 60 ~,m.
The composite material of the invention preferably
comprises at least one support comprising at least
aluminum, silicon, cobalt, manganese, zinc, vanadium,
molybdenum, indium, lead, bismuth, silver, gold,
nickel, copper, iron, titanium, platinum, stainless
steel, steel, brass, an alloy of these materials, or a
material coated with Au, Ag, Pb, Ti, Ni, Cr, Pt, Pd,
Rh, Ru and/or Ti.
The inorganic component present in the composite
material of the invention may comprise at least one
compound of at least one metal, semimetal or mixed
metal with at least one element from main groups 3 to 7
of the Periodic Table or at least one mixture of these
compounds. The compounds of the metals, semimetals or
mixed metals may comprise at least elements of the
transition group elements and from main groups 3 to 5
or at least elements of the transition group elements
or from main groups 3 to 5, these compounds having a
particle size of from 0.001 to 25 Vim. The inorganic
component preferably comprises at least one compound of
an element from transition groups 3 to 8 or at least
one element from main groups 3 to 5 with at least one
of the elements Te, Se, S, 0, Sb, As, P, N, Ge, Si, C,
Ga, Al or B, or at least one compound of an element
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from transition groups 3 to 8 and at least one element
from main groups 3 to 5 with at least one of the
elements Te, Se, S, 0, Sb, As, P, N, Ge, Si, C, Ga, A1
or B, or mixture of these compounds. With particular
preference, the inorganic component comprises at least
one compound of at least one of the elements Sc, Y, Ti,
Zr, V, Nb, Cr, MO, W, Mn, Fe, CO, B, Al, Ga, In, Tl,
Si, Ge, Sn, Pb, Sb or Bi with at least one of the
elements Te, Se, S, 0, Sb, As, P, N, C, Si, Ge or Ga,
such as Ti02, A1203, Si02, Zr02, Y203, BC, SiC, Fe309,
SiN, SiP, nitrides, sulfates, phosphides, silicides,
spinels or yttrium aluminum garnet, or one of these
elements itself. The inorganic component may also
comprise aluminosilicates, aluminum phosphates,
zeolites or partially exchanged zeolites, such as
ZSM-5, Na-ZSM-5 or Fe-ZSM-5, for example, or amorphous
microporous mixed oxides which may include up to 200 of
nonhydrolyzable organic compounds, such as, for
example, vanadium oxide-silicon oxide glass or aluminum
oxide-silicon oxide-methylsilicon sesquioxide glasses.
Preferably, at least one inorganic component lies
within a particle size fraction having a particle size
of from 1 to 250 nm or having a particle size of from
260 to 10,000 nm.
It may be advantageous for the composite material of
the invention to comprise at least two particle size
fractions of at least one inorganic component. It may
likewise be advantageous for the composite material of
the invention to comprise at least two particle size
fractions of at least two inorganic components. The
particle size ratio may be from 1:1 to 1:10,000,
preferably from 1:1 to 1:100. The quantitative ratio of
the particle size fractions in the composite material
may be preferably from 0.01:1 to 1:0.01.
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The permeability of the composite material of the
invention is limited to particles having a certain
maximum size by the particle size of the at least one
inorganic component used.
A feature of the composite material of the invention is
that it comprises at least one organic and/or inorganic
material which carries surface charges. This material
may be present in the form of an admixture in the
microstructure of the composite material.
Alternatively, it may also be advantageous for the
inner and/or outer surfaces of the particles present in
the composite material to be coated with a layer of an
organic and/or inorganic material which carries surface
charges.
Such layers have a thickness of from 0.0001 to 1 ~tm,
preferably a thickness of from 0.001 to 0.05 ~,tm.
In one particular embodiment of the composite material
of the invention, at least one organic and/or inorganic
material which carries surface charges is present in
the interparticulate volume of the composite material.
This material fills some or all, preferably some, of
the interparticulate volume.
The surfaces of the organic and/or inorganic materials
have ionic groups on which at least one polyelectrolyte
layer can be adsorbed.
It may be advantageous for the material which carries
surface charges to comprise ionic groups selected from
the group consisting of alkylsulfonic acid, sulfonic
acid, phosphoric acid, alkylphosphonic acid,
dialkylphosphinic acid, carboxylic acid,
tetraorganylammonium, organylsulfonium, organyl-
phosphonium and tetraorganylphosphonium groups or
mixtures of these groups having the same charge. These
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ionic groups may be organic compounds attached
chemically and/or physically to inorganic particles.
Preferably, the ionic groups are connected to the inner
and/or outer surface of the particles present in the
composite material by way of aryl and/or alkyl chains.
The material which carries surface charges in the
composite material may be an organic material, such as
a polymer, for example. Preference is given to polymers
containing strongly basic or strongly acidic functional
groups. With particular preference, this polymer
comprises a sulfonated polytetrafluoroethylene, a
sulfonated polyvinylidene fluoride, an aminolyzed
polytetrafluoroethylene, an aminolyzed polyvinylidene
fluoride, a sulfonated polysulfone, an aminolyzed
polysulfone, a sulfonated polyetherimide, an aminolyzed
polyetherimide, or a mixture of these polymers.
As the inorganic material which carries surface
charges, the composite material may comprise at least
one compound from the group consisting of oxides,
phosphates, phosphates, phosphonates, sulfates,
sulfonates, vanadates, stannates, plumbates, chromates,
tungstates, molybdates, manganates, titanates,
silicates, aluminosilicates and aluminates or mixtures
of these compounds of at least one of the elements A1,
K, Na, Ti, Fe, Zr, Y, Va, W, Mo, Ca, Mg, Li, Cr, Mn,
Co, Ni, Cu and Zn or mixtures of these elements.
Alternatively, the inorganic material which carries
surface charges may comprise at least one partially
hydrolyzed compound from the group consisting of
oxides, phosphates, phosphates, phosphonates, sulfates,
sulfonates, vanadates, stannates, plumbates, chromates,
tungstates, molybdates, manganates, titanates,
silicates, aluminosilicates and aluminates or mixtures
of these compounds of at least one of the elements Al,
K, Na, Ti, Fe, Zr, Y, Va, W, Mo, Ca, Mg, Li, Cr, Mn,
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Co, Ni, Cu and Zn or a mixture of these elements.
Preferably, the inorganic material which carries
surface charges in the composite material of the
invention is at least one amorphous and/or crystalline
compound, carrying groups some of which cannot be
hydrolyzed, of at least one of the elements Zr, Si, Ti,
Al, Y or vanadium or mixtures of these elements or
compounds.
The polyelectrolyte layer or polyelectrolyte coating
present on the inner and/or outer surfaces of the
composite material of the invention comprises
polyelectrolytes which carry negative and/or positive
charges. Preferably, the polyelectrolyte layer
comprises, in alternation, anionic and cationic or
cationic and anionic polyelectrolytes.
It may also be advantageous for the polyelectrolyte
layer to comprise at least one polyelectrolyte which
has anionic and cationic properties. Polyalphaamino-
acrylic acid, for example, may be such a
polyelectrolyte which has anionic and cationic
properties.
Preferably, the polyelectrolyte layer comprises at
least one polyelectrolyte from a group which embraces
polyallylamine hydrochloride, polyethyleneimine,
polyvinylamine, polyvinyl sulfate potassium salt,
polystyrenesulfonate sodium salt, and polyacrylamido-2-
methyl-1-propanesulfonic acid.
With very particular preference the polyelectrolyte
layer features a ratio of carbon atoms to possible ion
pair bonds of from 2:1 to 20:1, preferably from 4:l to
8:1. For example, a polyvinyl complex comprising
polyvinyl sulfate and polyvinylamine has a ratio of 4.
Heteroatoms that replace carbon atoms, like, say, the
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silicon in organosilicon compounds, are treated like
carbon atoms as far as forming the ratio is concerned.
The composite material of the invention may be
flexible. Preferably, the polyelectrolyte coated
composite material can be bent to a smallest radius of
5 mm, with particular preference to a smallest radius
of 1 mm.
The process of the invention for preparing a composite
material which carries a polyelectrolyte layer on the
inner and/or outer surfaces is described by way of
example below, without any intention to restrict the
process of the invention to this preparation.
The process of the invention for preparing a composite
material as claimed in at least one of claims 1 to 14,
comprises coating a composite material which has
surface charges and is based on at least one perforate
and permeable support comprising on at least one side
of the support and/or in the interior of the support at
least one inorganic component comprising substantially
at least one compound of a metal, semimetal or mixed
metal with at least one element from main groups 3 to
7, at least once with a polyelectrolyte.
The composite material which has surface charges is
obtainable in a variety of ways. Firstly, materials
which carry surface charges or materials which carry
surface charges following a further treatment may be
used in the preparation of the composite material.
Secondly, existing permeable composite materials may be
treated with materials which carry surface charges or
with materials which carry surface charges following a
further treatment.
The preparation of a composite material which has
surface charges may be carried out by means of a
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process for preparing a composite material based on at
least one perforate and permeable support comprising on
at least one side of the support and in the interior of
the support at least one inorganic component comprising
substantially at least one compound of a metal,
semimetal or mixed metal with at least one element from
main groups 3 to 7. This preparation process is
described in detail in PCT/EP98/05939.
In this process for preparing the composite material,
at least one suspension comprising at least one
inorganic component of at least one compound of at
least one metal, semimetal or mixed metal with at least
one of the elements from main groups 3 to 7 is brought
into and onto at least one perforate and permeable
support and by heating at least once the suspension is
solidified on or in, or on and in, the support
material.
In this process it may be advantageous to bring the
suspension onto and into, or else onto or into, at
least one support by means of printing, pressing,
injecting, rolling, knife coating, brushing, dipping,
spraying, or pouring.
The perforate and permeable support onto which or into
which, or else onto which and into which, at least one
suspension is brought may comprise at least one
material selected from carbon, metals, alloys,
ceramics, minerals, plastics, amorphous substances,
natural products, composites, composite materials, or
of at least one combination of these materials.
Permeable supports used may also include those which
have been made permeable by treatment with laser beams
or ion beams. The supports used are preferably woven
fabrics of fibers or wires of the above materials, such
as metal wovens or polymer wovens, for example.
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The suspension used, which may comprise at least one
inorganic component and at least one metal oxide sol,
at least one semimetal oxide sol or at least one mixed
metal oxide sol, or a mixture of these sols, may be
prepared by suspending at least one inorganic component
in at least one of these sols.
The sols are obtained by hydrolyzing at least one
compound, preferably at least one metal compound, at
least one semimetal compound or at least one mixed
metal compound, with at least one liquid, solid or gas.
In this context it may be advantageous for the liquid
used to be water, alcohol or an acid, for example, for
the solid used to be ice, or for the gas used to be
water vapor, or at least one combination of these
liquids, solids or gases. It may likewise be
advantageous for the compound to be hydrolyzed to be
added, prior to the hydrolysis, to alcohol or an acid
or combination of these liquids. The compound to be
hydrolyzed is preferably at least one metal nitrate,
metal chloride, metal carbonate, metal alkoxide
compound or at least one semimetal alkoxide compound,
with particular preference at least one metal alkoxide
compound, metal nitrate, metal chloride, metal
carbonate, or at least one semimetal alkoxide compound,
selected from the compounds of the elements Ti, Zr, A1,
Si, Sn, Ce and Y or from the lanthanoids and actinoids,
such as titanium alkoxides, such as titanium
isopropylate, for example, silicon alkoxides, zirconium
alkoxides, or a metal nitrate, such as zirconium
nitrate, for example.
It may be advantageous to carry out the hydrolysis of
the compounds to be hydrolyzed using at least half the
molar ratio of water, water vapor or ice, based on the
hydrolyzable group of the hydrolyzable compound.
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The hydrolyzed compound may be peptized by treatment
with at least one organic or inorganic acid, preferably
an organic or inorganic acid having a strength of from
to 60% and, with particular preference, with a
5 mineral acid selected from sulfuric acid, hydrochloric
acid, perchloric acid, phosphoric acid and nitric acid
or a mixture of these acids.
It is possible to use not only sols prepared as
10 described above but also commercial sols, such as
titanium nitrate sol, zirconium nitrate sol or silica
sol, for example.
It may be advantageous for at least one inorganic
component having a particle size of from 1 to 10,000 nm
to be suspended in at least one sol. Preferably, an
inorganic component comprising at least one compound
selected from metal compounds, semimetal compounds,
mixed metal compounds and metal mixed compounds with at
least one of the elements from main groups 3 to 7, or
at least one mixture of these compounds, is suspended.
With particular preference, at least one inorganic
component comprising at least one compound from the
oxides of the transition group elements or the elements
of main groups 3 to 5, preferably oxides selected from
the oxides of the elements Sc, Y, Ti, Zr, Nb, Ce, V,
Cr, Mo, W, Mn, Fe, Co, B, A1, In, Tl, Si, Ge, Sri, Pb
and Bi, such as, for example, Y203, Zr02, Fez03, Fe30q,
Si02 and A1203, is suspended. The inorganic component
may also comprise aluminosilicates, aluminum
phosphates, zeolites, including partially exchanged
zeolites, such as ZSM-5, Na-ZSM-5 or Fe-ZSM-5, for
example, or amorphous microporous mixed oxides, with or
without up to 200 of nonhydrolyzable organic compounds,
such as, for example, vanadium oxide-silicon oxide
glass or aluminum oxide-silicon oxide-methylsilicon
sesquioxide glasses.
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The mass fraction of the suspended component is
preferably from 0.1 to 500 times that of the hydrolyzed
compound used.
Through the appropriate choice of the particle size of
the suspended compounds as a function of the size of
the pores, holes or interstices of the perforate
permeable support, and also through the layer thickness
of the composite material of the invention and through
the proportional sol/solvent/metal oxide ratio, it is
possible to optimize the freedom from cracking in the
composite material.
When using a woven mesh having a mesh size of, for
example, 100 ~m it is possible to increase the freedom
from cracking by using, preferably, suspensions
comprising a suspended compound having a particle size
of at least 0.7 ~tm. In general, the ratio of particle
size to mesh size or pore size should be from 1:1000 to
50:1000. The composite material of the invention may
preferably have a thickness of from 5 to 1000 Vim, with
particular preference from 50 to 150 Nxn. The suspension
comprising sol and compounds to be suspended preferably
has a ratio of sol to compounds to be suspended of from
0.1:100 to 100:0.1, preferably from 0.1:10 to 10:0.1
parts by weight.
The suspension present on or in, or else on and in, the
support may be solidified by heating this assembly at
from 50 to 1000°C. In one particular embodiment of the
process, this assembly is exposed to a temperature of
50 to 100°C for from 10 minutes to 5 hours. In another
particular embodiment of the process of the invention,
this assembly is exposed to a temperature of from 100
to 800°C for from 1 second to 10 minutes, with
particular preference to a temperature of from 350 to
600°C for from 30 seconds to 4 minutes.
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The assembly may be heated by means of heated air, hot
air, infrared radiation, microwave radiation, or
electrically generated heat. In one particular
embodiment of the process of the invention it may be
advantageous for the assembly to be heated using the
support material as an electrical resistance heating
element. For this purpose the support may be connected
to a current source via at least two contacts of the
supports. Depending on the power of the current source
and the level of voltage emitted, the support heats up
when the current is switched on, and by means of this
heating the suspension present in and on its surface
may be solidified.
In a another, particularly preferred embodiment of the
process of the invention, the suspension may be
solidified by bringing it onto or into, or else onto
and into, a preheated support and so solidifying it
directly after application.
In accordance with the invention, the composite
material which carries surface charges may be obtained
by using at least one polymer-bound commercial Bronsted
acid or Bronsted base during the described preparation
of the composite material. Preferably, the composite
material which carries surface charges may be obtained
by using at least one sol which comprises
polyelectrolyte solutions or polymer particles which
carry fixed charges. It may be advantageous for the
polyelectrolytes or polymers which carry fixed charges
to have a melting point or softening point of below
500°C. Preferred polyelectrolytes or polymers which
carry fixed charges that are used comprise sulfonated
polytetrafluoroethylene, sulfonated polyvinylidene
fluoride, aminolyzed polytetrafluoroethylene,
aminolyzed polyvinylidene fluoride, sulfonated
polysulfone, aminolyzed polysulfone, sulfonated
polyetherimide, aminolyzed polyetherimide, or a mixture
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thereof. The fraction of the polyelectrolytes or of the
polymers which carry fixed charges in the sol that is
used is preferably from 0.0010 by weight to 50.00 by
weight, with particular preference from 0.010 by weight
to 250 by weight. During the production and processing
of the ion-conducting composite material, the polymer
may undergo chemical or physical changes, or chemical
and physical changes.
The composite material which carries surface charges
may also be obtained by using a sol which comprises at
least one material which carries surface charges or at
least one material which carries surface charges
following a further treatment, with said sol being used
during the preparation of the composite material. It is
preferred to add materials to the sol which lead to the
formation of inorganic layers which carry surface
charges on the inner and/or outer surfaces of the
particles present in the composite material.
In accordance with the invention, the sol may be
obtained by hydrolyzing at least one metal compound, at
least one semimetal compound or at least one mixed
metal compound, or a combination of these compounds,
with a liquid, a gas and/or a solid. As the liquid, gas
and/or solid for hydrolysis it is preferred to use
water, water vapor, ice, alcohol or acid, or a
combination of these compounds. It may be advantageous
to add the compound to be hydrolyzed to alcohol and/or
an acid prior to the hydrolysis. Preferably, at least
one nitrate, chloride, carbonate or alkoxide of a metal
or semimetal is hydrolyzed. With very particular
preference, the nitrate, chloride, carbonate or
alkoxide to be hydrolyzed is a compound of the elements
Ti, Zr, V, Mn, W, Mo, Cr, A1, Si, Sn and/or Y.
It may be advantageous if a compound to be hydrolyzed
carries nonhydrolyzable groups alongside hydrolyzable
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groups. As such a compound to be hydrolyzed it is
preferred to use an alkyltrialkoxy or dialkyldialkoxy
or trialkylalkoxy compound of the element silicon.
In accordance with the invention, at least one water
and/or alcohol soluble acid or base may be added to the
sol for preparing the composite material. It is
preferred to add an acid or base of the elements Na,
Mg, K, Ca, V, Y, Ti, Cr, W, Mo, Zr, Mn, A1, Si, P or S.
The sol used to prepare the material which carries
surface charges in accordance with the invention may
also comprise nonstoichiometric metal, semimetal or
nonmetal oxides and/or hydroxides produced by changing
the oxidation state of the corresponding element. The
oxidation state may be changed by reaction with organic
compounds or inorganic compounds or by means of
electrochemical reactions. Preferably, the change in
oxidation state is brought about by reaction with an
alcohol, aldehyde, sugar, ether, olefin, peroxide or
metal salt. Compounds having the ability to change
oxidation state in this way may be those, for example,
of Cr, Mn, V, Ti, Sn, Fe, Mo, W or Pb.
In accordance with the invention it may be advantageous
to add substances to the sol which lead to the
formation of inorganic structures which carry surface
charges. Examples of possible substances of this kind
include zeolite particles and/or (3-aluminosilicate
particles.
In this way it is possible in accordance with the
invention to prepare, for example, a permeable
composite material which carries surface charges
composed almost exclusively of inorganic substances. In
this context, a relatively high value must be placed on
the composition of the sol, since it is necessary to
use a mixture of different hydrolyzable components.
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These individual components must be carefully matched
to one another in terms of their hydrolysis rate. It is
also possible to produce the nonstoichiometric metal
oxide hydrate sols by means of corresponding redox
reactions. The metal oxide hydrates of the elements Cr,
M, V, Ti, Sn, Fe, Mo, W or Pb are very readily
accessible in this way. The compounds which carry
surface charges on the inner and outer surfaces are
then different, partially hydrolyzed or nonhydrolyzed
oxides, phosphates, phosphates, phosphonates,
stannates, plumbates, chromates, sulfates, sulfonates,
vanadates, tungstates, molybdates, manganates,
titanates, silicates or mixtures thereof of the
elements Al, K, Na, Ti, Fe, Zr, Y, Va, W, Mo, Ca, Mg,
Li, Cr, Mn, Co, Ni, Cu or Zn, or mixtures of these
elements.
In another preferred embodiment of the process of the
invention, existing permeable composite materials with
or without surface charges may be treated with
materials which have surface charges or with materials
which carry surface charges following a further
treatment. Such composite materials may be commercially
customary permeable materials or composite materials,
or else may be composite materials as described, for
example, in PCT/EP98/05939. It is, however, also
possible to use composite materials obtained by the
process described above.
In accordance with the invention, permeable composite
materials which have surface charges are obtained by
treating a composite material which has a pore size of
from 0.001 to 5 ~,m and no or an inadequate number of
surface charges with at least one material which
carries surface charges or with at least one material
which carries surface charges following a further
treatment.
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The treatment of the composite material with at least
one material which carries surface charges or with at
least one material which carries surface charges
following a further treatment may take place by
impregnating, dipping, brushing, roller application,
knife coating, spraying, or other coating techniques.
Following the treatment with at least one material
which carries surface charges or at least one material
which carries surface charges following a further
treatment, the composite material is preferably
thermally treated. This thermal treatment is conducted
with particular preference at a temperature from 100 to
700°C.
Preferably, the material which carries surface charges
or the material which carries surface charges following
a further treatment is applied to the composite
material in the form of a solution having a solvent
content of from 1 to 990. In accordance with the
invention, the material used to prepare the composite
material which has surface charges may comprise
polyorganylsiloxanes having at least one ionic
constituent. The polyorganylsiloxanes may comprise,
inter alia, polyalkyl- and/or polyarylsiloxanes and/or
further constituents.
It may be advantageous if the material used to prepare
the composite material which has surface charges
comprises at least one Bronsted acid or Bronsted base .
It may likewise be advantageous if the material used to
prepare the composite material which has surface
charges comprises at least one trialkoxysilane solution
or suspension containing acidic and/or basic groups.
Preferably, at least one of the acidic or basic groups
is a quaternary ammonium, phosphonium, alkylsulfonic
acid, carboxylic acid or phosphonic acid group.
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In this way, using the process of the invention, it is
possible for an existing permeable composite material,
for example, to be given surface charges
retrospectively by treatment with a silane. For this
purpose, a 1-20o solution of this silane in a water-
containing solution is prepared and the composite
material is dipped therein. Solvents used may be
aromatic and aliphatic alcohols, aromatic and aliphatic
hydrocarbons, and other common solvents or mixtures. It
is advantageous to use ethanol, octanol, toluene,
hexane, cyclohexane, and octane. After the adhering
liquid has dripped away, the impregnated composite
material is dried at about 150°C and, either directly
or following repeated subsequent coating and drying at
150°C, may be used as a permeable composite material
which has surface charges. Both silanes carrying
cationic groups and silanes carrying anionic groups are
suitable for this purpose.
It may further be advantageous for the solution or
suspension for treating the composite material to
comprise not only a trialkoxysilane but also acidic or
basic compounds and water. Preferably the acidic or
basic compounds include at least one Bronsted or Lewis
acid or base known to the skilled worker.
Alternatively, in accordance with the invention, the
composite material may be treated with solutions,
suspensions or sols comprising at least one material
which carries surface charges. This treatment may be
performed once or may be repeated a number of times.
With this embodiment of the process of the invention,
layers are obtained of one or more identical or
different, partially hydrolyzed or nonhydrolyzed
oxides, phosphates, phosphates, phosphonates, sulfates,
sulfonates, vanadates, tungstates, molybdates,
manganates, titanates, silicates or mixtures thereof of
the elements Al, K, Na, Ti, Fe, Zr, Y, Va, W, Mo, Ca,
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Mg, Li, Cr, Mn, Co, Ni, Cu or Zn or mixtures of these
elements.
In accordance with the invention, the composite
materials which have surface charges, obtained in
accordance with the invention by using materials which
carry surface charges or materials which carry surface
charges following a further treatment in the
preparation of the composite material, or by a
subsequent treatment of a composite material with
materials which carry surface charges or materials
which carry surface charges following a further
treatment are coated from 1 to 500 times, preferably
from 20 to 100 times, with at least one
polyelectrolyte.
The polyelectrolytes may be applied by spraying, knife
coating, rolling and/or dipping or similar processes.
The polyelectrolytes to be applied are preferably in a
solution. These solutions contain preferably from 0.001
to 2.0 monomol/1, with particular preference from 0.005
to 0.5 monomol/l, of the respective polyelectrolyte.
Suitable solvents include acids, preferably dilute
mineral acids, and with very particular preference
dilute hydrochloric acid. The solutions preferably
contain the respective polyelectrolyte in a
concentration of from 0.01 mmol/1 in a dilute
hydrochloric acid having a pH of about 1.7. For the
application it may be of advantage to add electrolytes,
such as NaCl, NaC104 or KCl, for example, to the
polyelectrolyte solution. As electrolytes it is
possible to use 1:1, 1:2 or 2:1 electrolytes, such as
KC1, MgClz or KZS04, for example. The ionic strength of
the electrolytes used in the polyelectrolyte solution
is preferably from 0.02 to 10.
Preferably, the composite material of the invention is
prepared by coating a composite material which carries
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surface charges in alternation with at least one
anionic polyelectrolyte and at least one cationic poly-
electrolyte. Where the polyelectrolytes used, i.e., the
polyanion and polycation, are the same in each dipping
operation, layers having the structure ABABAB etc, are
obtained. By varying the polyanions and/or polycations
in the dipping procedures, it is possible to obtain
layers having a structure ABCDABCD or else an irregular
structure.
The polyelectrolytes are preferably applied by means of
a simple dipping process. The composite material of the
invention is preferably prepared by coating a composite
material which carries surface charges in alternation
with at least one anionic polyelectrolyte and at least
one cationic polyelectrolyte. For this purpose the
composite material which carries surface charges is
dipped in alternation into solutions of cationic and
anionic polyelectrolytes. The first dipping process
must involve the formation of a first lamina onto which
a subsequent lamina may be adsorbed.
Where the surface of the composite material is equipped
with negative charges, the first dipping process of the
coating sequence comprises dipping into a solution
comprising a cationic polyelectrolyte; where the
surface of the composite material is equipped with
positive charges, the first dipping process of the
coating sequence comprises dipping into a solution
comprising an anionic polyelectrolyte.
Where the polyelectrolytes are applied by a dipping
process, it may be advantageous to leave the composite
material which carries surface charges and is to be
coated in the polyelectrolyte solution for about half
an hour. Following this dipping period, the composite
material is preferably washed at least twice with water
before a subsequent dipping process.
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In each of the following dipping steps, a virtually
monomolecular lamina of the respective polyelectrolyte
is deposited on the surface of opposite charge. The
conformation of the deposited polyelectrolyte depends
greatly on whether low molecular mass salts, such as
NaCl, for example, are added as electrolytes to the
polyelectrolyte solution. Without the addition of
electrolyte, the polyelectrolytes are deposited in an
approximately expanded conformation; with the addition
of electrolyte, they are deposited in a clustered
conformation. By depositing polyelectrolytes in the
clustered conformation it is possible to obtain thicker
polyelectrolyte layers. The thickness of the deposited
layer is therefore much greater with addition of
electrolyte than without. The bonding between the
polyelectrolytes is attributable exclusively to
physical interactions between the polyelectrolytes. By
far the greatest attracting force is the interaction
between the differently charged ionic groups of the
polyelectrolytes. The most important influencing
variable on the pervaporation performance in the case
of polyelectrolyte membranes is the charge density;
that is, the number of carbon atoms per charge.
Polyelectrolytes used for the process of the invention
are preferably those where the polyelectrolyte layer
obtained has a ratio of carbon atoms to possible ion
pair bonds of from 2:1 to 20:1, preferably from 4:1 to
8:1, silicon atoms in polyelectrolyte layers comprising
organosilicon polyelectrolytes being counted like
carbon atoms.
As polyelectrolytes for preparing the composite
material of the invention it is preferred to use
polyelectrolytes such as, for example, poly(allylamine
hydrochloride), poly(ethyleneimine), polyvinylamine,
polyvinyl sulfate potassium salt, poly(2-acryloamido-
2-methyl-1-propanesulfonic acid), polyacrylic acid,
cellulose sulfate potassium salt, chitosan,
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poly(4-vinylpyridine), poly(styrenesulfonate) sodium
salt, and dextran sulfate sodium salt.
As cationic polyelectrolytes it is possible in
particular to use polyallylamine hydrochloride,
polyethyleneimine and/or polyvinylamine, for coating.
Anionic polyelectrolytes used are preferably
polyacryloamido-2-methyl-1-propanesulfonic acid and/or
polyvinyl sulfate potassium salt.
The polyelectrolyte coated permeable composite
materials of the invention as claimed in any of
claims 1 to 14 are highly suitable for use in
separating substances by pervaporation and vapor
permeation. With particular preference, the composite
materials of the invention may be used as membranes in
pervaporation.
Of particular importance is the separation of water and
ethanol by pervaporation. For the use of the composite
material of the invention as a membrane it is possible,
for example, to separate water from ethanol with a
separation factor of up to 500 in the case of a flow
rate through the membrane of up to 11,000 g/mzh, a
temperature of about 80°C and a pressure difference of
about 1 bar. The incoming material contained between 3
and 18o water in ethanol.
A further principal field of application of the
polyelectrolyte coated composite material of the
invention is its use as a membrane in solvent drying,
since in this application the membrane materials
employed at present are frequently limited, owing to
the swelling behavior of the support polymers and their
relatively low thermal stability, to a few solvents
(ethanol and the like) and to temperatures below 80°C.
Using the composite material of the invention as a
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membrane, it is also possible to dewater solvents such
as, for example, THF, methylene chloride or acetone.
The greater thermal stability of the polyelectrolyte
coated permeable composite materials of the invention
allows them to be used, furthermore, in pervaporation
at temperatures higher than those in processes
according to the present state of the art, such as the
treatment of component streams in the context of a
rectification. The massive technical advantage in this
case is that the component streams to be treated need
no longer be passed through heat exchangers but instead
can be passed directly to the pervaporation membrane at
the respective process temperature (which may be up to
110°C), at which point a vapor permeation is frequently
carried out as well. In other words, the incoming
stream is passed in the vapor state over the membranes.
The polyelectrolyte coated composite materials of the
invention are also suitable as membranes for such
applications owing to the increased temperature
stability in relation to conventional polyelectrolyte
membranes.
The values plotted in Figs. 1 to 4 are measurements
obtained when using a membrane of the invention in the
separation of ethanol/water mixtures. Figs. 1 and 3
show the permeate flow as a function of the initial
water content in the ethanol/water mixture of the feed.
Figs. 2 and 4 show the water content in the permeate,
in o by weight, as a function of the initial water
content in the ethanol/water mixture of the feed.
The measurements on which Figs . 1 and 2 are based were
obtained in the course of conducting the experiment
from Example 3c in which an experimental temperature of
about 80°C was set. The measurements on which Figs. 3
and 4 are based were obtained in the course of
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conducting the experiment from Example 3c in which the
experimental temperature was from about 105 to 110°C.
The polyelectrolyte coated composite materials of the
invention, the process for preparing them, and their
use are described by means of the following examples,
without being restricted thereto.
Example 1.1 Preparation of a composite material as
per PCT/EP98/05939
a) 120 g of titanium tetraisopropoxide were stirred
vigorously with 140 g of deionized ice until the
resultant precipitate was very finely divided.
Following the addition of 100 g of 25o strength
hydrochloric acid, stirring was continued until
the phase became clear, and 280 g of a-aluminum
oxide of the type CT3000SG from Alcoa,
Ludwigshafen, were added, and the mixture was
stirred for a number of days until the aggregates
broke up. This suspension was subsequently applied
in a thin layer to a stainless steel mesh with a
mesh size of 90 dun and was solidified within a
very short time at 550°C.
b) 40 g of titanium tetraisopropoxide were hydrolyzed
with 20 g of water and the resulting precipitate
was peptized with 120 g of nitric acid (250
strength). This solution was stirred until it
clarified, and following the addition of 40 g of
titanium dioxide from Degussa (P25) stirring was
continued until the agglomerates broke up. After a
further 250 ml of water had been added to the
suspension, it was applied to a porous support
(prepared in accordance with Example 1.1a) and
solidified within a very short time at
approximately 500°C.
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Example 1.2 Preparation of an ionic composite
material
a) An inorganic permeable composite material as per
Example 1.1 b was dipped into a solution of the
following components: 5% Degussa Silan 285 (a
propylsulfonic acid-triethoxysilane), 20o DI water
in 75o ethanol. Prior to use it was necessary to
stir the solution at room temperature for 1 hour.
After excess solution had been allowed to drip away,
the composite material was dried at from 80°C to 150°C
and then used.
b) An inorganic permeable composite material as per
Example 1.1 b was dipped into a solution of the
. following components: 5o Dynasilan 1172 from
Degussa-Hiils, 2.5% hydrochloric acid (350
strength); 30o ethanol and 62.50 DI water. Prior
to use it was necessary to stir the solution at
room temperature for 30 minutes.
After excess solution had been allowed to drip away,
the composite material was dried at from 80°C to 150°C
and then used.
c) 20 g of aluminum alkoxide and 17 g of vanadium
alkoxide were hydrolyzed with 20 g of water and
the resulting precipitate was peptized with 120 g
of nitric acid (25o strength). This solution was
stirred until it clarified and, following the
addition of 40 g of titanium dioxide from Degussa
(P25), was stirred until all of the agglomerates
broke up. Following adjustment of the pH to about
6, the suspens,'_on was applied in a layer 100 ~m
thick to an E-glass cloth type 1675 from CS-
Interglas and dried at 500°C within 1 minute. This
gave a composite material furnished with negative
fixed charges.
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d) 20 g of tetraethyl orthosilicate and 17 g of
potassium permanganate were hydrolyzed with 20 g
of water and reduced completely with 6o strength
hydrogen peroxide solution. The resulting
precipitate was partially peptized with 100 g of
sodium hydroxide solution (25% strength). This
solution was stirred for 24 hours and, following
the addition of 40 g of titanium dioxide from
Degussa (P25), was stirred until all of the
agglomerates broke up. After the pH had been
adjusted to about 8, the suspension was applied to
a permeable support having a pore size of about
0.1 ~m (from Atech, Essen). This support was then
dried at 500°C within 1 minute. This gave a
composite material furnished with negative fixed
charges.
Example 2 Polyelectrolyte coated composite
material
a) A composite material made ionic in accordance with
1.2a was coated with polyelectro lytes, the coating
taking place by dipping, with one side of the
membrane being masked off, so that coating was
effected on one side only. To this end the
composite material was first immersed for 30
minutes in a solution of polyethyleneimine
(0.01 monomol/1 in aqueous HCl, pH 1.7) and then
cleaned by twofold immersion in water. The
composite material was then immersed for 30
minutes in a solution consisting of 0.01 monomol/1
polyvinyl sulfate potassium salt (in
aqueous HC1,
pH 1.7) and subsequently washed twice with water.
The dipping operation in the polyethyleneimine
solution was then repeated. The alternate
immersion in the polyethyleneimine and
the
polyvinyl sulfate sodium salt so lution was carried
out 60 times per solution. The membrane was
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subsequently dried in a circulating-air drying
cabinet at 90°C for 24 h and was suitable for use
as a membrane in a pervaporation cell.
b) In accordance with Example 2a, composite materials
made ionic in accordance with Example 1.2a were
coated with different polyelectrolytes, coating
taking place by dipping with one side of the
membrane being masked off so that coating was
effected on one side only. The membranes thus
prepared were used for pervaporation. The
pervaporation took place at a temperature of
58.5°C and at a pH of 1.7. An ethanol/water
mixture having a water content of 6.2o by weight
was used. Table 1 lists the polyelectrolyte
solutions used in each case with the compounds
used as polycations or polyanions, respectively,
the number of dipping cycles, and also the flow
data, water contents of the permeate, and
separation factors. All of the membranes or
polyelectrolyte coated composite materials
prepared in this way are suitable for use as
pervaporation membranes for separating ethanol and
water or for removing water from organic solvents.
c) In accordance with Example 2a, composite materials
made ionic in accordance with Example 1.2a were
coated with different polyelectrolytes, coating
taking place by dipping with one side of the
membrane being masked off so that coating was
effected on one side only. In a deviation from
Example 2a, both polyelectrolyte solutions
additionally contained NaCl at a concentration of
1 mol/1. The membranes thus prepared were used for
pervaporation. The pervaporation took place at a
temperature of 58.5°C and at a pH of 1.7. An
ethanol/water mixture having a water content of
6.2% by weight was used. Table 1 again lists the
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polyelectrolyte solutions used in each case with
the compounds used as polycations or polyanions,
respectively, the number of dipping cycles, and
also the flow data, water contents of the
permeate, and separation factors. All of the
membranes or polyelectrolyte coated composite
materials prepared in this way are suitable for
use as pervaporation membranes for separating
ethanol and water or for removing water from
organic solvents.
d) A composite material made ionic in accordance with
1.2a was coated with polyelectrolytes, the coating
taking place by dipping, with one side of the
membrane being masked off, so that coating was
effected on one side only. To this end the
composite material was first immersed for 30
minutes in a solution of polyvinylamine
(0.01 monomol/1 in aqueous HC1, pH 1.7) containing
NaC104 in a concentration of 1 mol/1 and then
cleaned by twofold immersion in water. The
composite material was then immersed for 30
minutes in a solution consisting of 0.01 monomol/1
polyvinyl sulfate potassium salt (in aqueous HC1,
pH 1.7) likewise containing NaC104 in a
concentration of 1 mol/1 and subsequently washed
twice with water. The dipping operation in the
polyvinylamine solution was then repeated. The
alternate immersion in the polyvinylamine and the
polyvinyl sulfate sodium salt solution was carried
out 30 times per solution, so that 60 layers were
applied to the composite material. The membrane
was subsequently dried in a circulating-air drying
cabinet at 90°C for 24 h and was suitable for use
as a membrane in a pervaporation cell.
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Table l: Polyelectrolyte solutions used in
Experiments 2b and 2c, number of dipping
cycles, flow data, water contents of the
permeate, and separation factors.
Polycatio Polyanion Number Flow HZOPermeatea
of
n dipping [g/mzh] [o by wt.]
cycles
PEI PVS 60 159 61.6 24.3
PVAM PVS 60 316 70.3 35.8
PAH PAMSA 60 216 62.0 24.7
PVAM + PVS + 30 693 51.1 15.8
(1 mol/1 (1 mol/1
NaCl) NaCl)
PVAM + PVS + 45 308 77.3 51.6
(1 mol/1 (1 mol/1
NaCl) NaCl)
PVAM + PVS + 60 210 91.0 153
(1 mol/1 (1 mol/1
NaCl) NaCl)
Key:
PEI: Poly(ethyleneimine)
PVS: Polyvinyl sulfate potassium salt)
PVAM: Poly(vinylamine)
20
PAMSA: Poly(2-acrylamido-2-methyl-1-propanesulfonic
acid)
PAH: Poly(allylamine hydrochloride)
The separation factor a is the ratio of the composition
of the permeate (p) to the composition of the feed (f),
i.e..
a = ( [H20]p/ [ethanol]p) / ( [H20] f/ [ethanol] fj
Example 3 Use examples
a) Using the polyelectrolyte coated composite
material prepared in accordance with Example 2a it
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was possible to separate a mixture of 94o ethanol
and 6o water. The flow through the polyelectrolyte
coated composite material used as membrane was
159 g/m2h, with an ethanol content of about 30 to
40o in the permeate. The temperature of the
retentate was 58.5°C and the permeate pressure was
mbar.
b) Using a polyelectrolyte coated composite material
10 prepared in accordance with Example 2c using
polyvinylamine as polycation and polyvinyl sulfate
as polyanion, the same mixture as in Example 3a
was separated under the same temperature
conditions. The flow was 210 g/m2h, with an
15 ethanol content in the permeate of 9%.
c) Using a polyelectrolyte coated composite material
prepared in accordance with Example 2d, different
mixtures of water and ethanol were separated at a
temperature of 80°C. Fig. 1 is a plot of permeate
flow as a function of water content in the mixture
to be separated (feed). Fig. 2 is a plot of the
water content of the permeate as a function of the
water content in the mixture to be separated.
It is clearly evident that at a temperature of 80°C an
initial mixture (feed) containing about 5o water and
about 95% ethanol is separated, with a permeate flow of
about 2000 g/m2h, such that the permeate has a water
content of about 88% and an ethanol content of about
12%.
d) The experiment from Example 3c was repeated at a
temperature of from 105 to 110°C. Fig. 3 is a plot
of permeate flow as a function of water content in
the mixture to be separated (feed). Fig. 4 is a
plot of the water content of the permeate as a
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function of the water content in the mixture to be
separated.
It is clearly evident that at a temperature of 105 to
110°C an initial mixture (feed) containing about 5.50
water and about 94.50 ethanol is separated, with a
permeate flow of about 4000 g/m2h, such that the
permeate has a water content of about 92o and an
ethanol content of about 8%.
